Apodisation of coherent optical systems

Thesis (Ph.D.)--University of Rochester. College of Engineering and Applied Science. Institute of Optics, 1979. This thesis was digitized by the Institute of Optics in 2014 and was determined to have lapsed into the public domain. If you are the author and have questions about the digitization of your work, please contact Kari Brick, Graduate Program Coordinator for the Institute of Optics, at kbrick@ur.rochester.edu. Other contact information for the Institute is available at http://www.optics.rochester.eduThe widespread and increasing use of lasers has generated a need for optical systems which are designed to be used with coherent light. The basis of optical design was developed at a time when most light sources produced incoherent light and thus many common optical design procedures tacitly assume that the system will be used with incoherent light. Because the coherence properties of the light can effect the performance of a system, a reevaluation of some classical design criteria is necessary. Many design considerations are important; however, this work concentrates on finite apertures and how they affect the performance of systems to be used with coherent light. The purpose of this thesis is to determine the effects of finite apertures in coherent systems and to investigate apodisation as a technique to improve system performance for systems which have undesirable finite aperture effects. The coherent systems investigated can be separated into two groups by their function. The formation of an image is the primary function of the first group. The main function of the second group is the propagation of light energy from source to receiving plane. The different groups are called imaging and beam propagation systems, respectively. Finite aperture effects and apodisation are considered for both types of systems. The effect of finite apertures on the spatial invariance of several common coherent imaging systems is developed and it is shown that many of these systems exhibit spatially variant behavior which is aperture dependent. Configurations are suggested which produce spatially invariant performance for the systems considered and those results are generalized to apply to arbitrary imaging systems. Several apodisation functions have been suggested for use with coherent imaging systems. These apodisations are evaluated and compared. It is shown that an upper limit exists on a real apodisation function if the corresponding amplitude impulse response of the apodised system is to be real and everywhere positive. A class of apodisation functions which exceed the limit is evaluated and general conclusions are given regarding its usefulness in coherent imaging systems. Also, complex apodisations are considered and it is concluded that they will not be useful in the general case. Beam propagation systems suffer from Fresnel diffraction effects when the beam is truncated by an aperture. The nonuniform beam profiles produced by Fresnel diffraction are illustrated and it is shown that these nonuniformities can be controlled by apodisation. A general criterion is developed which determines the necessary apodisation function to produce a uniform beam profile for a specified propagation. It is also shown that the effects of diffraction in appropriately apodised systems are small and thus a determination of the beam profile can be made from the principles of geometrical optics. A photographic technique for the fabrication of apodisation filters is discussed in detail and compared with other techniques. It is concluded that although the photographically produced filters have a low intensity damage threshold, they have an advantage in cost and versatility. Examples of apodisation filters produced are presented. Finally, the results of laboratory simulations of both coherent imaging and beam propagation systems are presented and compared with theoretical predictions. The agreement between experiment and theory is very good